Showing papers on "Energy (signal processing) published in 1986"

Accurate and simple density functional for the electronic exchange energy: Generalized gradient approximation

Abstract: The electronic exchange energy as a functional of the density may be approximated as ${E}_{x}[n]={A}_{x}\ensuremath{\int}{d}^{3}r{n}^{\frac{4}{3}}F(s)$, where $s=\frac{|\ensuremath{ abla}n|}{2{k}_{F}n}$, ${k}_{F}={(3{\ensuremath{\pi}}^{2}n)}^{\frac{1}{3}}$, and $F(s)={(1+1.296{s}^{2}+14{s}^{4}+0.2{s}^{6})}^{\frac{1}{15}}$. The basis for this approximation is the gradient expansion of the exchange hole, with real-space cutoffs chosen to guarantee that the hole is negative everywhere and represents a deficit of one electron. Unlike the previously publsihed version of it, this functional is simple enough to be applied routinely in self-consistent calculations for atoms, molecules, and solids. Calculated exchange energies for atoms fall within 1% of Hartree-Fock values. Significant improvements over other simple functionals are also found in the exchange contributions to the valence-shell removal energy of an atom and to the surface energy of jellium within the infinite barrier model.

Free energy simulations.

Abstract: Monte Carlo or molecular dynamics simulations involve the numerical determinations of the statistical thermodynamics and related structural, energetic and (in the case of molecular dynamics) dynamic properties of an atomic or molecular assembly on a high-speed digital computer. Applications to molecular systems range from the study of the motions of atoms or groups of atoms of a molecule or macromolecule under the influence of intramolecular energy functions to the exploration of the structure and energetics of condensed fluid phases such as liquid water and aqueous solutions based on intermolecular potentials. The quantities determined in a typical Monte Carlo or molecular dynamics simulation include the average or mean configurational energy (thermodynamic excess internal energy), various spatial distribution functions for equilibrium systems, and time-correlation functions for dynamical systems, along with detailed structural and energetic analyses thereof. Diverse problems in structural and reaction chemistry of molecules in solution, such as solvation potentials, solvent effects on conformational stability and the effect of solvent on chemical reaction kinetics and mechanism via activated complex theory also require a particular knowledge of the configurational free energy, which in principle follows directly from the statistical thermodynamic partition function for the system. Considerations on free energy in molecular simulations take a distinctly different form for intramolecular and intermolecular degrees of freedom. For the intramolecular case, the problem involves vibrational and librational modes of motion on the intramolecular energy surface. We will discuss briefly a t the end of this paper the harmonic and quasiharmonic approximation used to compute vibrational contributions to the free energy, but we will restrict the focus herein to the intermolecular case, where the particles of the system undergo diffusional motion and a harmonic or quasiharmonic treatment breaks down. These considerations apply also in the case of a flexible molecule, where conformational transitions are effectively an intramolecular “diffusional mode.” Conventional Monte Carlo and molecular dynamics procedures for diffusional modes, although firmly grounded in Boltzmann statistical mechanics and dynamics, do not proceed via the direct determination of a partition function because of well-known

Monte Carlo study of electronic transport in Al 1 − x Ga x As/GaAs single-well heterostructures

Abstract: A study of electronic transport in ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ga}}_{\mathrm{x}}$As/GaAs single-well structures including multisubband conduction at 77 and 300 K has been performed. The electronic states of the quantum well are calculated self-consistently taking the five lowest subbands into account. The numerically obtained wave functions and energy levels are used to obtain the major two-dimensional scattering rates in each subband. Polar optical- and acoustic-phonon (via deformation-potential) scattering are considered including intersubband transitions. For ionized impurity scattering, the screening effects due to the five lowest subbands are taken into account to obtain the Fourier-transformed Coulomb potential. The steady-state and transient behavior of the electrons in the well are studied through a Monte Carlo particle simulation. It is shown that high transient velocities [(3--8)\ifmmode\times\else\texttimes\fi{}${10}^{7}$ cm/sec] can be expected at low and intermediate fields.

Time-resolved spectroscopy of the visible emission band in strontium titanate

Abstract: The luminescence spectrum of ${\mathrm{SrTiO}}_{3}$ from 1.6 to 3.2 eV has been studied as a function of time elapsed after excitation. A broad emission band with maximum intensity at 2.44 eV can be observed. The shape of this band, including some fine structure, can be reproduced by the Huang-Rhys model for excitation-lattice interaction. A vibron energy \ensuremath{\Elzxh}\ensuremath{\Omega}=88 meV and a Huang-Rhys factor ${S}_{0}$\ensuremath{\simeq}6 are deduced. It is shown that those values are consistent with what is required for exciton self-trapping. The luminescence intensity has been followed for times between 100 ns and 10 ms. The decay curves indicate that two recombination processes are involved. We associate the first one with self-trapped excitons interacting with acoustic phonons and the second with the retarded formation of self-trapped excitons from localized electrons and holes. The emission appears to be quenched by a nonradiative recombination channel whose activation energy is 0.07 eV.

Multiple-scattering regime and higher-order correlations in x-ray-absorption spectra of liquid solutions

Abstract: By making a comparison between the Mn K-edge absorption of (${\mathrm{MnO}}_{4}$${)}^{\mathrm{\ensuremath{-}}}$ and [Mn(${\mathrm{H}}_{2}$O${)}_{6}$${]}^{2+}$ complexes in aqueous solution we obtain an experimental determination of the energy extent of the type-II multiple-scattering (MS) regime that is substantially wider than expected. Theoretical calculations based on the MS formalism support this conclusion. We also recognize three energy regions in the absorption spectra of these complexes: a full MS region, where numerous or an infinite number of MS paths of high order contribute (depending on whether the MS series converges or not), an intermediate MS region, where only a few MS paths of low order are relevant, and a single-scattering region where the photoelectron is backscattered only once by the ligands [extended x-ray-absorption fine-structure (EXAFS) regime]. Theoretical considerations show that this must be a general situation in x-ray-absorption spectra and opens the way to a unified scheme for their interpretation. The energy extent of the three regions is obviously system dependent. We also show how to generalize to MS contributions the usual EXAFS analysis using curved-wave propagators and indicate how to extract geometrical information from the spectra of the two clusters investigated. In particular the method is used to derive the Mn---O---O---Mn path length in the (${\mathrm{MnO}}_{4}$${)}^{\mathrm{\ensuremath{-}}}$ complex.

Multiple source DF signal processing: An experimental system

Abstract: The multiple signal characterization (MUSIC) algorithm is an implementation of the signal subspace approach to compute parameter estimates of multiple point-source signals from the observed voltages received on an array of M antennas. In it, the solution to the multiple source direction finding (DF) problem is provided by the intersection of the signal subspace (obtained from the received data) and the array manifold (obtained via array calibration or prior knowledge of array directional characteristics). The MUSIC algorithm was implemented to experimentally verify the performance of the signal subspace approach to DF under very general scenarios and conditions which are regarded as difficult to impossible in traditional systems. The results of those experiments are described herein. The experimental system consisted of an eight-element antenna array 13 wavelengths in diameter, an eight-channel receiver and digitizer, and a minicomputer with disk storage to process the digitized data. With ideal instrumentation, the MUSIC algorithm provides performance that, as the amount of data collected increases without limit, is asymptotically ideal. However, with finite precision and finite data collection, the performance of even an ideal system can be a sensitive function of source and scenario parameters. Tests demonstrated the resolution of three sources all within one beamwidth (5 \deg ), even when the closer two were spaced less than 0.2 beamwidths. Sources that were polarized differently could be resolved at closer spacings. Experimental DF accuracy was limited by the oncalibrated scattering of source energy from the test range support tower and from the ground. The measured direction of arrival of one source changed by less than 0.01 beamwidths as the other two sources were switched on and off in all combinations. In general, results indicated that all parameters of a source can be measured and the signal waveform can be recovered as well in the presence of other sources less than a beamwidth away.

Monte Carlo evaluation of trial wave functions for the fractional quantized Hall effect: Disk geometry.

Abstract: Monte Carlo methods have been employed to evaluate the energy of two previously proposed trial wave functions for the quasiparticle at the $\ensuremath{ u}=\frac{1}{3}$ quantized Hall state of the two-dimensional electron system. The two wave functions have the same energy within our statistical accuracy, and are consistent with a value ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\varepsilon}}}_{+}(\frac{1}{3})\ensuremath{\approx}\frac{(0.073\ifmmode\pm\else\textpm\fi{}0.008){e}^{2}}{\ensuremath{\epsilon}{l}_{o}}$, where ${l}_{0}$ is the magnetic length, and $\ensuremath{\epsilon}$ the background dielectric constant. Simulations of the quasihole state confirm previous estimates of ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\varepsilon}}}_{\ensuremath{-}}(\frac{1}{3})\ensuremath{\approx}\frac{0.026{e}^{2}}{\ensuremath{\epsilon}{l}_{0}}$. We have also studied the charge distributions of the quasiparticle and quasihole states, and we have evaluated the energies of a previously proposed microscopic trial wave function for the ground state at $\ensuremath{ u}=\frac{2}{5}, \frac{2}{3}, \mathrm{and} \frac{2}{7}$.

Binding and formation energies of native defect pairs in GaAs

Abstract: The formation energies of native lattice-site point defects and of nearest-neighbor lattice-site defect pairs in GaAs have been calculated by use of the self-consistent Green's-function technique From these results, we deduce a binding energy of \ensuremath{\sim}2\ifmmode\pm\else\textpm\fi{}1 eV for the pairs ${V}_{\mathrm{As}+{V}_{\mathrm{Ga}}}$, ${\mathrm{As}}_{\mathrm{Ga}+{\mathrm{Ga}}_{\mathrm{As}}}$, ${\mathrm{As}}_{\mathrm{Ga}+{\mathrm{V}}_{\mathrm{As}}}$, and ${\mathrm{Ga}}_{\mathrm{As}+{\mathrm{V}}_{\mathrm{Ga}}}$ We also obtain end-point energy differences for the vacancy migration hops ${V}_{\mathrm{Ga}\ensuremath{\rightarrow}{V}_{\mathrm{As}+{\mathrm{As}}_{\mathrm{Ga}}}}$ and ${V}_{\mathrm{As}\ensuremath{\rightarrow}{V}_{\mathrm{Ga}+{\mathrm{Ga}}_{\mathrm{As}}}}$ For these latter reactions, violent donor-acceptor transitions occur and the Fermi energy determines the stable form

Density-functional theory for excited states in a quasi-local-density approximation.

Abstract: The starting point of this paper is a recent extension by Theophilou of the Hohenberg-Kohn-Sham (HKS) density-functional theory to ensembles of systems consisting of the M lowest eigenstates, equally weighted. As in the HKS theory the key quantities are the exchange-correlation energy, ${E}_{\mathrm{xc}{}^{M}[\mathrm{n}(\mathrm{r})]}$, and potential, ${v}_{\mathrm{xc}{}^{M}(\mathit{r};[\mathit{n}(\mathit{r}\mathcal{'})])}$. The present paper provides expressions for these quantities, valid for systems of slowly varying density. Even for such systems, however, there are essential nonlocal effects. Nevertheless both ${E}_{\mathrm{xc}{}^{M}}$ and ${v}_{\mathrm{xc}{}^{M}}$ can be calculated in terms of quantities characteristic of appropriate uniform thermal ensembles. This theory is the analog of the ground-state local-density approximation and allows calculation of excited-state energies and densities.

Atomic-number dependence of soft-x-ray emission from various targets irradiated by a 0.53- microm-wavelength laser

Abstract: The spectrum-resolved radiant energies in the 0.1--1.6 keV range from various plane targets irradiated by a 0.53-\ensuremath{\mu}m laser at 0.1--1.0-nsec pulses with intensities of ${10}^{13}$--2\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{cm}}^{2}$ are obtained. The conversion efficiency increases with the laser-pulse duration. The physical meaning of the obtained spectrum, such as the correspondence between the emitted photons and the responsible plasma density and temperature region, is clarified from the computational studies by using the one-dimensional hydrodynamic Lagrangian code which is coupled with a nonlocal thermodynamic equilibrium average ion model and multigroup radiation transport. The energy transport of the absorbed laser energy is discussed. Atomic-number dependences of the obtained spectrum and x-ray conversion efficiency show an undulatory structure. This is well reproduced qualitatively and to some extent quantitatively by the code calculation and the contributions of the electronic transitions in different orbital shells are approximately estimated.

Recombination at dangling bonds and steady-state photoconductivity in a -Si:H

Abstract: A simple model of recombination at dangling bonds in a-Si:H is proposed to explain the steady-state photoconductivity and \ensuremath{\gamma}-exponent variations with the equilibrium Fermi-level position. The appropriate statistics for correlated defects and the Shockley-Read formalism are used to obtain a parametrical representation of photoconductivity versus optical generation rate. Oscillations of \ensuremath{\gamma} between 0.5 and 1 when ${E}_{F}$ is shifted in the central region of the gap depend mainly on the density of dangling bonds and the energy positions of the singly (${T}_{3}^{0}$) and doubly (${T}_{3}^{\mathrm{\ensuremath{-}}}$) occupied levels. Experimental results on lightly-boron-doped glow-discharge a-Si:H are in agreement with the model and give a location of the ${T}_{3}^{0}$ level at 0.95 eV from ${E}_{c}$, an effective correlation energy of 0.4 eV, and a ratio of charge-to-neutral-state capture cross sections of 50. Finally, the dangling-bond-state occupation probabilities are shown to be weakly modified by illumination even at high photon fluxes. Consequences for the interpretation of ESR experiments are also discussed.

Periodicity of the Aharonov-Bohm effect in normal-metal rings.

Abstract: We study the Aharonov-Bohm effect in single normal-metal rings and show that averaging the conductance over many energies is equivalent to ensemble averaging. Thus raising the temperature T above a crossover ${\mathrm{T}}_{\mathrm{c}}$ changes the flux periodicity of magnetoresistance oscillations from h/e to h/2e. ${\mathrm{T}}_{\mathrm{c}}$ is determined by an energy correlation range, hD/${\mathrm{L}}^{2}$. The persistence of the h/e oscillations to high fields is explained.

Tunneling through indirect-gap semiconductor barriers.

Abstract: Tunneling-current measurements in ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}\ensuremath{-}\mathrm{GaAs}\ensuremath{-}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ heterostructures under hydrostatic pressure show that nonresonant tunneling occurs preferentially through the lowest potential barrier, while resonant tunneling is determined solely by a $\ensuremath{\Gamma}$-point profile. For fixed voltages, the low-temperature current through a 100 \AA{}-40 \AA{}-100 \AA{} structure with ${\mathrm{Ga}}_{0.60}$${\mathrm{Al}}_{0.40}$As barriers increased with pressure, up to 11 kbar. The rate of increase showed an abrupt rise at \ensuremath{\sim}4 kbar, which is attributed to tunneling through a $\ensuremath{\Gamma}\ensuremath{-}X$ barrier. This interpretation is consistent with a rapid increase of the tunneling current in AlAs-GaAs-AlAs, even at low pressures. On the other hand, a negative-resistance feature associated with resonant tunneling via quantum-well states, shifted smoothly to lower voltages with pressure, indicating that the energy of the confined states is established by a pressure-dependent $\ensuremath{\Gamma}$-point profile.

Explicit estimation of ground-state kinetic energies from electron densities.

Abstract: This paper discusses some initial steps toward the goal of finding explicit procedures for calculating, to a good approximation, the minimum kinetic energy consistent with a given particle density \ensuremath{\rho}(r) for a system of fermions. The strategy proposed begins by separating the desired kinetic energy into a sum ${T}_{W}$+${T}_{\mathrm{theta}}$, where ${T}_{W}$\ensuremath{\ge}0 is the Weizsaaumlcker energy F ${d}^{3}$r \ensuremath{\Vert}\ensuremath{ abla}\ensuremath{\rho}${\ensuremath{\Vert}}^{2}$/8\ensuremath{\rho} , and ${T}_{\mathrm{theta}}$\ensuremath{\ge}0. Approximations are applied to ${T}_{\mathrm{theta}}$ alone, and are sought in the form of interpolations that will be nearly correct in two limits: small departures from uniform density, for which exact results are known from linear-response theory, and cases where a region containing no more than one particle of a given spin becomes isolated from the rest of the distribution by regions of nearly vanishing density. Only highly nonlocal functionals can behave properly in either of these limits. A few other conditions for satisfactory approximations to ${T}_{\mathrm{theta}}$ are noted. Some explicit interpolation formulas are offered for one-dimensional problems, and are tested on a variety of examples; one such is found to give kinetic energies to within a few percent in nearly all cases examined. More detailed tests are possible by comparing correct and approximate potentials yielding a given density, or correct and approximate densities for the ground state of a given potential; tests on the position dependence of kinetic energy density, however, are physically meaningless. A few remarks are offered on the additional problems that beset extension of the scheme to three dimensions; foremost among these is that of computational tractability.

Synchrotron radiation study of Cd1-xMnxTe (0 <= x <= 0.65).

Abstract: The electronic structure of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te (0\ensuremath{\le}x\ensuremath{\le}0.65) has been investigated by photoemission in the photon energy range from 20 to 140 eV. A sharp (\ensuremath{\approxeq}1 eV full width at half maximum) peak located 3.4 eV below the valence-band maximum (VBM) is assigned to emission from Mn 3${d}_{\ensuremath{\uparrow}}$ states with e symmetry. The ${t}_{2}$ components hybridize significantly with the Te 5p states and contribute therefore nearly uniformly to the top 6 eV of the valence bands. Weak structures below 6 eV of mixed Mn 3d--Te 5p character occur also due to the p-d hybridization. Cd and Te 4d core-level binding energies remain constant over the whole range of Mn concentrations when measured relative to the VBM. This implies that there are negligible chemical shifts and that the VBM is not affected by the replacement of Cd by Mn. The increase in the optical gap with x is thus due to an increase of the conduction-band energy, in agreement with a shift in the Te 4d absorption threshold as measured by partial-yield spectroscopy. A maximum of the Te 5p component in the density of empty conduction states is identified \ensuremath{\approxeq}2 eV above threshold. The Mn 3p\ensuremath{\rightarrow}3${d}_{\ensuremath{\downarrow}}$ excitations are atomiclike. The results are interpreted in terms of a schematic linear-combination-of-atomic-orbitals level scheme for the band structure of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te.

Energy spectra of donors and acceptors in quantum-well structures: Effect of spatially dependent screening.

Abstract: The energy spectra of shallow donors and acceptors in GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As quantum-well structures have been calculated. The binding energies of the impurities were obtained within a variational calculation in the effective-mass approximation. Calculations were performed for simple neutral and double singly ionized impurities as functions of the position of the impurity in a GaAs quantum well of infinite depth and for various slab thicknesses. The effect of the spatially dependent screening is modeled with a dielectric response of the form ${\ensuremath{\epsilon}}^{\mathrm{\ensuremath{-}}1}$(r)=${\ensuremath{\epsilon}}_{0}^{\mathrm{\ensuremath{-}}1}$+(1-${\ensuremath{\epsilon}}_{0}$ $^{\mathrm{\ensuremath{-}}1}$)${e}^{\mathrm{\ensuremath{-}}r/a}$, with a screening parameter a\ensuremath{\approxeq}1.1 a.u. characteristic of bulk GaAs. Results are compared with Bastard's theory, which is based on a constant-${\ensuremath{\epsilon}}_{0}$ screening, and it is found that spatially dependent screening effects are small for donors down to very thin slab thicknesses, but can be quite important for all acceptors in GaAs quantum wells over a large range of slab thicknesses. Calculated results with improved statistics are in quantitative agreement with experimental data on neutral donors and acceptors.

Helium and neon photoelectron satellites at threshold

Abstract: Photoionization of helium and neon to excited satellite states, ${\mathrm{He}}^{+}$ nl and ${\mathrm{Ne}}^{+}$ 1${s}^{2}$2${s}^{2}$2${p}^{4}$nl, was studied with synchrotron radiation and threshold electron analysis. Photoelectron satellites have been directly measured at threshold for the first time to our knowledge. The relative satellite cross sections were determined over the kinetic energy range from 0 to 1 eV. The angular distributions were also evaluated close to threshold. Strong correlation effects were observed in two cases. For He near threshold, the angular-distribution asymmetry parameter \ensuremath{\beta} is near zero for the n=2 satellite and is increasingly negative for the higher-n satellites, in agreement with the theoretical prediction of Greene. In the threshold photoelectron spectrum of Ne, many final states are present, some with quartet spin multiplicity and others with high-L values.

Energy transfer in the plasma wake-field accelerator.

Abstract: The transfer of energy from the driving beam to the trailing beam in the plasma wake-field accelerator is studied in computer simulations. We show that with an asymmetric current distribution in the driving bunch, trailing particles can gain energies up to ${(1+k_{p}^{}{}_{}{}^{2}{Z}^{2})}^{\frac{1}{2}}\ensuremath{\Delta}\ensuremath{\gamma}m{c}^{2}$, where $Z$ is the bunch length and $\ensuremath{\Delta}\ensuremath{\gamma}m{c}^{2}$ the average energy loss of driving electrons. Because of the relative phase slippage and the two-stream instability, the process of energy gain degrades before the driving beam loses all of its energy; however, even for ${\ensuremath{\gamma}}_{i}=150$, $\frac{\ensuremath{\Delta}\ensuremath{\gamma}}{{\ensuremath{\gamma}}_{i}}\ensuremath{\gtrsim}70%$, with an energy gain \ensuremath{\sim} 1 GeV. Transverse effects are briefly discussed.

Adaptive filter for dual energy radiographic imaging

Abstract: A radiographic scanner (A) generates a high energy image representation which is stored in a high energy image matrix (V) and a low energy image representation which is stored in a low energy image memory (U). A pair of filter functions selecting circuits (C) select a first or soft tissue specific filter function and second or bone specific filter function, respectively. The soft tissue filter function selecting circuit selects and adjusts the soft tissue filter function in accordance with the pixel value of the low energy image representation for each corresponding pair of pixel values. Convolvers (44, 46) convolve pixel values from the high and low energy image representations with the selected and adjusted filter functions. A soft tissue transform function (48) transforms the filtered high and low energy image representations into a soft tissue or other material specific image representation (42). The other filter selecting and adjusting circuit selects and adjusts the bone specific filter functions which are convolved with the high and low energy image representations by convolvers (54, 56). A bone specific transform function (58) transforms the filtered high and low energy image representations into a bone basis image.

Exactly exponential band tail in a glassy semiconductor

Abstract: Transient-photocurrent measurements in glassy ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ reveal an accurate power-law behavior spanning more than eight decades of time. In the multiple-trapping model this requires a density of valence-band-tail states varying exponentially with energy over almost five decades. The accuracy of this exponential and its continuation deep into the gap are inconsistent with existing models for band tails based on short-range potential fluctuations, but show a remarkable degree of perfection in the electronic structure of this ostensibly random material.

1/R expansion for H 2 + : Calculation of exponentially small terms and asymptotics

Abstract: The energy of any bound state of the hydrogen molecule ion ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$ has an expansion in inverse powers of the internuclear distance R of the form Rayleigh-Schr\"odinger perturbation theory (RSPT) gives the coefficients ${E}^{(N)}$ but is otherwise unable to treat the exponentially small series, which in part are characteristic of the double-well aspect of ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$. (Here n denotes the hydrogenic principal quantum number.) We develop a quasisemiclassical method for solving the Schr\"odinger equation that gives all the exponentially small subseries.The RSPT series diverges: for the ground state ${E}^{(N)}$\ensuremath{\sim}-(N+1)!/${e}^{2}$ for large N. The ${E}^{(N)}$ asymptotics are governed via a dispersion relation by the imaginary ${e}^{\mathrm{\ensuremath{-}}2R/n}$ series, which itself is given by the square of the ${e}^{\mathrm{\ensuremath{-}}R/n}$ series times a ``normalization integral.'' That the expansion itself contains imaginary terms might seem inconsistent with the reality of the ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$ eigenvalues. In fact, the RSPT series is Borel summable for R complex. The Borel sum has a cut on the real R axis, and its limit from above or below the positive R axis is complex. The imaginary ${e}^{\mathrm{\ensuremath{-}}2R/n}$ (and higher) series consist of just the counterterms to cancel the imaginary part of the Borel sum.Extensive numerical examples are given. Of interest is a weak (down by a factor ${N}^{\mathrm{\ensuremath{-}}6}$) alternating-sign contribution to ${E}^{(N)}$, which is uncovered both theoretically and numerically. Also of interest is the identification of the Borel sum of the RSPT series with nonphysical boundary conditions. This too is illustrated both theoretically and numerically.

Microscopic wave functions for the fractional quantized Hall states at nu =2/3 and 2/7.

Abstract: New microscopic trial wave functions are proposed for fractional quantized Hall states at $\ensuremath{ u}=\frac{2}{(2p+1)}$. Results for $\ensuremath{ u}=\frac{2}{5} \mathrm{and} \frac{2}{7}$ based on Monte Carlo and exact numerical computations are presented. For $N\ensuremath{\le}10$ electrons on a sphere, energies differ from the exact ground-state energy by less than 0.3%. For even $p$, such as $\ensuremath{ u}=\frac{2}{5}$, the trial wave function may be regarded as a microscopic realization of the condensed quasiparticle state postulated in hierarchical schemes.

Raman scattering and optical-absorption studies of the metastable alloy system GaAs x Sb 1 − x

Abstract: The optical properties of ${\mathrm{GaAs}}_{\mathrm{x}}$${\mathrm{Sb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ alloys grown across the entire concentration range by multitarget sputter deposition are reported. X-ray diffraction, optical absorption, and Raman spectra show the samples to be single-crystal, single-phase alloys for all x, including those in the miscibility gap. Alloy lattice constants are found to vary linearly with concentration. The direct \ensuremath{\Gamma}-point energy gaps, determined from optical-absorption measurements, show significant negative bowing. Contrary to previous reports, the Raman spectra exhibit two-mode behavior throughout, including a local mode of As in GaSb and a resonant mode of Sb in GaAs. An analysis of peak frequencies and line shapes versus concentration is given in the context of disorder effects. We observe broadenings much less severe and asymmetric than those seen in similar systems and usually interpreted in terms of k\ensuremath{ e}0 density-of-states activation. The observed adherence to zone-center selection rules for all x, suggests a more accurate interpretation to be one involving the k\ensuremath{\approxeq}0 spectral projection of the density of states.

Transport and relaxation of hot conduction electrons in an organic dielectric.

Abstract: Effective mean free paths of hot electrons in the energy range 0.5 eV\ensuremath{\le}${E}_{\mathrm{kin}\mathrm{\ensuremath{\le}}20}$ eV are determined experimentally for the paraffin n-${\mathrm{C}}_{36}$${\mathrm{H}}_{74}$ with the internal photoemission for transport analysis method. The hot-electron transport parameters are discussed in terms of fundamental scattering mechanisms in organic dielectrics. The influence of hot-electron-induced trap formation on the transport properties is investigated. The consequences for dielectric breakdown are pointed out.

ADPCM coder-decoder including partial band energy transition detection

Abstract: In an ADPCM coder and decoder including a so-called locking-unlocking adaptation speed control, the adaptation speed is locked to a very slow, almost constant, speed of adaptation for voiceband data and partial band energy signals, i.e., tones and tone like signals, and is unlocked to achieve a fast speed of adaptation for speech. When a so-called partial band energy signal is being inputted, the adaptation speed is biased toward the unlocked state and when a transition occurs from a partial band energy signal to another such signal, the adaptation speed is set to the totally unlocked state and coefficients of an adaptive predictor are set to prescribed values. This is done in both the coder and decoder to minimize generation of impulse noise in the decoder output.

Sulfur 1s core-level photoionization of SF6

Abstract: Photoelectron spectra have been taken between 2460 and 2600 eV photon energy across the discrete and continuum resonances in the vicinity of the sulfur K edge in gaseous ${\mathrm{SF}}_{6}$. Results at the below-threshold S 1s\ensuremath{\rightarrow}6${t}_{1u}$ resonance indicate that ``highly excited'' S 2p and S 2s satellites (with two core holes) are the primary autoionization final states of ${\mathrm{SF}}_{6}$${\mathrm{}}^{+}$. An observed asymmetric profile in the S(LVV) Auger angular distribution suggests interference effects in the alignment of these resonantly produced ${\mathrm{SF}}_{6}$${\mathrm{}}^{+}$ ions. Decay of the low-energy S 1s continuum resonances near 2507 eV photon energy into S 2p, S 2s, and/or valence photoemission channels indicates autoionizing character. These features are assigned as doubly excited states, leading to S 1s satellite thresholds observed here for the first time. At higher photon energies, between 2520 and 2570 eV, large oscillations in the S 1s cross section are reproduced well by multiple-scattering X\ensuremath{\alpha} calculations, but are not explained adequately by single-scattering plane-wave extended x-ray absorption fine structure effects. We speculate that improvements in the description of both the electron scattering process and the molecular potential are necessary to model the diffractive and nondiffractive (barrier interaction) effects in this energy region.

Paramagnetic neutron scattering from the Heisenberg ferromagnet EuO.

Abstract: Paramagnetic neutron scattering from the insulating Heisenberg ferromagnet EuO has been studied through the entire Brillouin zone. The measured linewidth \ensuremath{\Gamma} of the quasielastic scattering at ${T}_{c}$ follows the predictions of dynamic scaling \ensuremath{\Gamma}=${\mathrm{Aq}}^{2.5}$ up to momentum transfers q=0.4 A${\r{}}^{\mathrm{\ensuremath{-}}1}$. The new results disagree with an early work by Passell et al., where a smaller exponent and smaller linewidths have been reported, but agree perfectly with recent spin-echo measurements performed by Mezei at very small q. The ${q}^{2.5}$ dependence of \ensuremath{\Gamma} is now established over nearly four decades in energy. At large momentum transfers the energy distribution evolves into a three-peaked structure with much more spectral weight near E=0, in disagreement with the two-peaked structure reported by Mook. The cross sections near the zone boundary are in quantitative agreement with recent calculations by Young and Shastry. We conjecture that peaks at finite energy occur in EuO and EuS because of the short-ranged exchange interactions.

Stability of equilibrium for a nonlinear hyperbolic system describing heat propagation by second sound in solids

Abstract: We consider here the propagation of heat in a unidimensional case in which the only nonzero component of the heat flux is its x-component, q, and not only q, but also the absolute temperature, θ, and the internal energy density, e, are functions of x and the time, t. When, as we assume here, such a unidimensional flow of heat is not accompanied by appreciable deformation, the law of balance of energy reduces to $$ {{e}_{t}}{\text{ + }}{{q}_{x}}{\text{ = 0}} $$ (1.1) (with the subscripts t and x indicating partial derivatives), and the constitutive assumptions behind Fourier’s theory of heat conduction reduce to $$ q{\text{ = }} - x\left( \theta \right){{\theta }_{x}} $$ (1.2) and $$ e{\text{ = }}{{e}_{{\text{0}}}}\left( \theta \right),\quad {\text{i}}{\text{.e}}{\text{.,}}\quad {{e}_{t}} = {{c}_{0}}\left( \theta \right)\theta t\quad {\text{with}}\quad {{c}_{{\text{0}}}}\left( \theta \right){\text{ = }}{{e'}_{0}}\left( \theta \right), $$ (1.3) where x and c 0 are positive-valued functions characterizing the material under consideration. Thus, in Fourier’s theory, the evolution of q and θ is governed by the following system of equations: $$ \begin{gathered} q\;{\text{ + }}\;x\left( \theta \right)\theta x\;{\text{ = }}\;{\text{0, }} \hfill \\ qx\;{\text{ + }}\;{{c}_{{\text{0}}}}\left( \theta \right){{\theta }_{t}}\,{\text{ = }}\,{\text{0}}{\text{.}} \hfill \\ \end{gathered} $$ (1.4)

Feeding of electrical energy to circuits on a wheel for a tire-monitoring device

Abstract: A tire-monitoring device comprises, on a wheel of a vehicle, a converter stage which processes an electric signal representing measurements of the pressure and/or temperature of the tire in pulse form and a stage for transmitting the signal to the vehicle by inductive coupling. In order to minimize the consumption of electric energy by circuit components on the wheel, a solid state switch responsive to the transmission of energy from the vehicle to the wheel disconnects the circuit components on the wheel from the energy source. Also, in order to further minimize the consumption of electrical energy, only the positions of the ascending and descending fronts of the electric signal pulses representing the measurements are transmitted to the vehicle and the transmission coil is connected to the source of energy only when the pulse signals have ascending or descending fronts.

Acoustic attenuation due to domain walls in anisotropic superconductors, with application to U1-xThxBe13.

Abstract: We propose an explanation of the large sound-attenuation peak observed at the lower transition, ${T}_{c2}$, in superconducting ${\mathrm{U}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}{\mathrm{Be}}_{13}$. It assumes that the transition is between two different anisotropic superconducting phases, and that the low-temperature phase is tetragonally distorted. The domain-wall energy is very small at ${T}_{c2}$ and the attenuation due to the motion of walls is very large. This attenuation depends strongly on direction and polarization, and so the model may be easily tested. The implications for the phase diagram of these alloys are discussed.